1. Field of the Invention
The invention relates generally to development of latent electrostatic images on electrophotographic films, and particularly, to a method and apparatus which provides good image development for microfiche film over a wide latitude of exposure conditions and film sensitivity ranges.
2. Description of the Prior Art
In electrophotography, it is common to apply a uniform electrostatic charge to the surface of the photoconductive layer of a film. The charge is then selectively dissipated in a pattern by exposing the surface to a light image. The resulting pattern of charges produces an electrostatic latent image on the photoconductive layer, which is then rendered visible by applying thereto electrostatically charged developer particles which adhere to the surface of the photoconductive layer by electrostatic forces. A permanent visible image can be obtained, for example, by using developer particles which can be heat fused to the photoconductive layer, and subjecting them to a heat application step.
A major difficulty in prior art systems is that, to produce a visible image of sufficient intensity, undesirable light and dark areas are sometimes present due to distortion of the electric lines of force at the charge surface of the photoconductive layer. Prior efforts to reduce this effect have been through the use of a development electrode spaced from the surface of the film being developed and connected to a source of electric potential to provide a bias that affects the charged developer particles. Other efforts have employed the development electrode in an electrically floating mode.
Examples of prior art uses of development electrodes can be found in such patents as U.S. Pat. No. 3,249,088 to Ostensen, U.S. Pat. No. 3,611,982 to Coriele et al, and U.S. Pat. No. 3,655,419 to Tamai et al. The Ostensen patent discloses a developing tank unit for electrostatic printing. A carrier sheet or film is passed through the developing tank unit where charged pigment particles attach to the electrostatically charged film. Within the developing tank unit is an "intensifier member" in the form of a plate or a roller of conductive material which is electrically floating.
The Coriele et al. patent discloses a xerographic developing apparatus with three development electrodes along the path which the charged surface moves. The charge potential or magnitude on the various electrodes is varied (through complicated electrical circuitry in the preferred embodiments) to vary the concentration and positioning of the toner particles in the flow stream and to regulate the degree of development and cleaning obtained in each of the electroded regions.
The Tamai et al patent discloses an electrophotographic reversal developing process. The electrophotographic element is placed close to a floating development electrode in a container of electrostatically charged particles to induce on the electrode a potential which causes an electric field to occur at the image or information portions of the electrostatic latent image oppositely directed from that occurring at the background portions. Toner particles having a charge of the same polarity as that of the background portion are introduced between the development electrode and the electrophotographic element to develop the imaged portion.
While the development electrodes disclosed in these patents are beneficial for some uses, they are not entirely satisfactory for the type of developing which must take plce in a preferred type of microphotographic recording apparatus in which imaging and developing takes place automatically within a single apparatus. In this preferred form of microphotographic recording apparatus, data from successive documents is sequentially recorded on individual frames of a microfiche, and each frame is developed before imaging the next succeeding frame. Development is preferably accomplished by positioning the imaged frame against an opening in a developing chamber, and electrostatically charged toner particles are flowed through the developing chamber to contact the imaged frame.
In the preferred microphotographic recording apparatus, described above, there is need for flexibility in the developing process because of variations which occur in imaging-light intensity, variations in the degree of darkness of documents being copied, and variations in the properties of the photoconductors used in the process. All of these factors can result in the electrostatic charge potentials which remain on the film after imaging being higher or lower place desired, which in turn affects the attractive forces upon the toner particles during development. Of particular importance is the difference between the highest and lowest charge potential on the imaged film. Where this difference is very slight, there is a tendency to produce dark, or unclean, background areas in the image. Additionally, where the lowest charge potential on the imaged film is greater in magnitude than zero, there is a tendency for the toner particles to also deposit in the areas having that lowest charge potential. In the preferred type of microphotographic recording apparatus, the background of the documents being copied usually correspond to the lowest charge potential on the film, and that area on the film is seldom discharged to zero, due to photoconductor properties and/or practical limitations on imaging-light duration and intensity.
The use of development electrodes could be helpful in giving flexibility to the electrostatic development process and solving some of the above-described problems, but none of the prior art development electrodes are entirely satisfactory for use in the preferred microphotographic recording apparatus, nor were they used in the same manner. For example, the apparatus of Ostensen and of Tamai et al. have development electrodes which are submerged in a reservoir of developing particles along with the charged film for development. Furthermore, the apparatus of Ostensen, like the apparatus of Coriele et al, passes the charged film or plate through a developing station when developing, while in Tamai et al., the charged film remains stationary along with the developing particles within the reservoir. Probably the biggest difference in the use of these prior art development electrodes from any proposed use in the preferred apparatus is that they were designed to develop an entire film or member at one time, whereas only a single frame on a larger microfiche is developed at one time in the preferred microphotographic recording apparatus.
Particular problems occur when using development electrodes to develop images smaller than the entire film, particularly if the electrode is electrically floating. Those problems are due to the effect on the electrode of that portion of the microfiche beyond the charged frame area being developed, and thus were not recognized in the prior art. The development electrode, being a flat plate closely spaced from the electrophotographic element, forms a capacitor not only with the portion of the electrophotographic element which was charged and is being developed, but also with the surrounding portions of the electrophotographic element, which has an adverse effect on the development electrode to be explained below. Biased development electrodes such as the type described in the Coriele et al patent, which are biased through complex electrical circuitry, could overcome this effect, but the use of complex electrical circuitry is undesirable from a cost standpoint and also because of size limitations in the preferred type of apparatus.
In recognition of the disadvantages of the prior art, it is an object of the invention to provide a method and apparatus for developing electrostatic latent images on individual frames of a multi-frame microfiche in a manner which is simple and permits flexibility.